1. Field of the Invention
The present invention relates to a method for manufacturing nanopowders and, more particularly, to a method for manufacturing oxide powders in a nanometer scale through direct current plasma thermal reactions.
2. Description of Related Art
It is known that nanopowders are referred to as ultra-fine solid particles of size less than 100 nanometers. Generally speaking, the structure of nanopowders mainly includes two parts. The first part comprises particles with a diameter ranging from several nanometers to less than a hundred nanometers. The other part comprises the surfaces of the particles illustrated above or the interfaces between these particles. Basically, the former part has an either short- or long- range ordered structure. In contrast, the second part has a random structure without a long- or short-ranged order. Owing to the high surface area and special structure illustrated above, nanopowders often show different magnetic properties, dielectric properties, superconductivity, optical properties, and thermodynamic properties.
Conventionally, nanoparticles can be prepared through a top-down method such as high-energy mechanical milling, or a bottom-up method such as gas phase condensation, liquid phase chemical precipitation, and sol-gel. In general, the cost for preparing nanoparticles through liquid phase is low. However, serious agglomeration and low purity are the major problems.
Recent research reports disclose gas-phase condensation methods for preparing nanopowders. For example, a method for preparing metal oxide nanopowders is disclosed in U.S. Pat. No. 5,460,701. The nanopowders are prepared by introducing an electrical current to a cathode and an anode of a metal rod first. Then a plasma arc is generated in the gap between the anode and the cathode. The metal rod is vaporized soon and the vapor is condensed into powders of metal oxides through the assistance of inert gas flow and/or oxygen flow. However, metal rods have to be supplied in time before the electrode is consumed in this method for preparing nanopowders continuously.
Another method for preparing nanopowders can be seen in WO 9705994, wherein the nanopowders are prepared by heating a precursor of a target metal close to supercritical temperature for vaporization. Then the vaporized precursors are sprayed out to form a reaction mixture vapor. Subsequently, nanopowders are formed by passing the reaction mixture vapor through plasma. However, only precursors in solution form can be applied. In addition, the precursors require heating to a high temperature close to the supercritical temperature in this method.
More methods for preparing nanopowders are described in several patent documents. For example, in U.S. Pat. No. 5,876,688, zinc oxide nanopowders with a diameter less than 80 nm are prepared by injecting nebulized organic salts of zinc to a flame or plasma ranging from 250° C. to 2000° C. In U.S. Pat. No. 5,958,361, a nebulized ceramic precursor solution containing a volatile organic solvent and at least one glycolato polymetallooxanei is pyrolyzed in a flame to produce oxide nanopowders. In U.S. Pat. No. 6,099,696, nanocarbon materials are made by carrying carbon materials or carbon hydrogen compounds into plasma generated between a set of non-transferred electrodes through hydrogen gas.
Since pretreatment of the compounds of a target metal or supplying/replacing an electrode is required for preparing nanopowders, the recent methods illustrated above are not convenient enough for mass-producing nanopowders. Hence, a method for producing nanopowders with less pretreatment to mitigate and/or obviate the aforementioned problems is needed.